Definition: The range of activity within which assumptions about variable and fixed costs are valid.
Why It Matters
Cost behaviour assumptions (variable = constant per unit, fixed = constant total) only hold within a band of activity. Push past that band, and the relationships break.
The Capacity Example
| Monthly Rent | Machine Capacity | Tests/Month | Cost/Test |
|---|---|---|---|
| $8,000 | 2,000 tests | 500 | $16 |
| $8,000 | 2,000 tests | 2,000 | $4 |
| $16,000 | 4,000 tests | 2,500 | $6.40 |
At 2,001 tests, you need a second machine → step change in fixed costs.
Relevant range for the first machine: 0–2,000 tests
Connection to Economics
| Accounting Concept | Economics Equivalent |
|---|---|
| Relevant range | One short-run average cost curve |
| Exceeding relevant range | Moving to different SRAC curve |
| Long-run adjustments | Moving along LRAC envelope |
The relevant range IS the short run—where at least one input is fixed.
Practical Implications
| Planning Scenario | Relevance |
|---|---|
| Budgeting within current capacity | Safe to use existing cost relationships |
| Forecasting beyond capacity | Must account for step changes |
| CVP analysis | Only valid within relevant range |
Common Trap
Extrapolating cost relationships beyond the range where they were observed. A cost that’s fixed at 10,000 units may not be fixed at 100,000 units.
North: Where this comes from
- Variable vs Fixed Costs (the assumptions this range validates)
- Short-Run Cost Curves (economics foundation)
East: What opposes this?
- Long-Run Cost Curves (all inputs variable)
- Step Costs (what happens at range boundaries)
South: Where this leads
- Cost-Volume-Profit Analysis (valid within relevant range)
- Capacity Planning (what happens at boundaries)
- Break-Even Analysis (assumes relevant range)
West: What’s similar?
- Ceteris Paribus (assumptions hold “other things equal”)
- Model Assumptions (all models have validity boundaries)